In many manufacturing processes, there is a need to drill a matched hole through two or more parts of a workpiece that are to be subsequently joined together by passing a fastener through the matched hole. For example, in the manufacture of aircraft structures, there is frequently a need to fasten stringers to other parts such as paddle fittings. In some circumstances, one of the parts to be joined may have pilot holes (i.e., holes that are located in the desired locations where fasteners are to be placed, but that are smaller in diameter than the fasteners) already formed in it, while the other part has no holes. Once the parts are positioned in abutting relation to each other in the positions in which they are to be fixed, a drilling apparatus is aligned with the pilot hole and is operated to drill a full-size hole through the parts.
In such a drilling process, difficulties are encountered in terms of fixing the parts so they do not move relative to each other during the drilling of the hole, and in terms of properly aligning the rotary cutter of the drill such that the cutter is advanced through the workpiece in the intended direction, which typically is normal to the surfaces of the workpiece. The drilling apparatus must also be as compact as possible so that it can fit into tight spaces between portions of the aircraft structure.
Various types of portable drilling devices having clamping mechanisms have been developed for both clamping and drilling workpiece parts. For example, U.S. Pat. No. 5,888,032 describes a paddle fitting tool for clamping and drilling a forked paddle fitting and wing spar. The clamping mechanism of the device employs a nut runner coupled to a drill by a saddle. The nut runner is operable to clamp the paddle fitting and wing spar so that the drill can drill a hole through the wing spar that is concentric with holes that are already formed through two forks of the paddle fitting between which the wing spar is disposed. A locator pin on one of the clamping members engages in the pre-formed hole of one of the forks so as to locate the apparatus in the proper location for drilling. The locator pin has a diameter that is equal to that of the drill bit, and is retractable into the clamping member so that the drill bit can drill through the hole in the paddle fitting fork if this hole is initially undersize. The device described in the U.S. Pat. No. 5,888,032 patent has several limitations. First, the nut runner assembly is bulkier, heavier, and more expensive than the air cylinders that are typically used for clamping mechanisms in paddle fitting tools. The operation of the nut runner is also relatively slow in comparison with an air cylinder, which is an important consideration in view of the very large number of holes that must be drilled in a typical aircraft structure. Additionally, the device is awkward in use because the feed engagement lever, which must be activated to cause feeding of the drill bit to drill a hole, is not located in an ergonomically friendly location in relation to where an operator would most naturally grasp the apparatus in use. Furthermore, experience with a commercial embodiment of a paddle fitting tool made in accordance with the '032 patent has shown that the drill bit tends to be damaged at a relatively high rate. The reasons for the damage are not fully known, but it is suspected that one cause may be flexing of the clamp plates as a result of high clamping force produced by the nut runner, which causes the drill bit to become misaligned relative to the locator pin. Yet another drawback of the device of the '032 patent is that cut chips produced during a drilling operation fly in all directions, making clean-up more difficult. A further disadvantage is that the clamping mechanism can be unclamped while the drill motor is operating, which can be a safety hazard.
In other types of paddle fitting tools, as noted above, air cylinders are used for providing the clamping force. Air cylinders offer several advantages in terms of small size, light weight, high speed of operation, and low cost. However, one drawback in prior drilling and clamping devices using air cylinders for providing the clamping force is that if for any reason the air cylinder ceases to operate, which can be caused for instance by a loss in air pressure, the clamping mechanism will stop functioning. This can be hazardous because the apparatus can unexpectedly fall from the workpiece. Accordingly, it would be desirable to provide a drilling and clamping apparatus that uses an air cylinder or the like for clamping but that avoids this known drawback.